Electrolyte for use in a galvanic cell

ABSTRACT

Disclosed is a stable and environmentally compatible explosion-safe electrolyte for use in a galvanic cell. The electrolyte comprises salts of the composition ABL 2 , wherein A is lithium or a quarternary ammonium ion, B is boron, and L is a bidentate ligand which is bound to the central boron atom by two oxygen atoms. Described are the production and the electrochemical properties of several compositions.

This is a national stage application of PCT/EP94/01366 filed Apr. 29,1994.

This is a national stage application of PCT/EP94/01366 filed Apr. 29,1994.

BACKGROUND OF THE INVENTION AND FIELD OF THE INVENTION

The present invention concerns new electrolytes for use in galvaniccells, in particular lithium cells. In this paper a lithium cell isdefined as follows: Essentially a lithium cell consists of an anode, acathode, and an electrolyte. The anode consists of lithium, a lithiumalloy, or a compound which is able to intercalate lithium ions, forexample carbon. In general the cathode consists of a substance which isable to intercalate lithium ions or reacts electrochemically withlithium ions, where the potential of the cathode is markedly higher thanthe potential of the anode. The electrolyte consists of one or moresalts which are dissolved in a suitable solvent, a mixture of solvents,a polymer, or a mixture of a polymer with one or more solvents. Adistinction is made between primary and secondary lithium cells.

This definition includes the so-called `rocking chair batteries`. Recentinvestigations in this field are described in a review of B. Scrosati(J. Electrochem. Soc. Vol. 139, 2776, (1992)).

Typical solvents currently in use include:

Organic carbonates such as propylene carbonate (PC) or ethylenecarbonate (EC). Linear and cyclic ethers and polyethers such asdimethoxyethane (DME), diethoxyethane (DEE), tetrahydrofurane (THF),2-methyltetrahydrofurane (2-Me-THF), dioxolane, and the polymerpolyethylene oxide as well.

Typical salts currently in use include:

Lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), lithiumhexafluorophosphate (LiPF₆) lithium hexafluoroarsenate (LiAsF₆), lithiumtrifluoromethylsulfonate (LiCF₃ SO₃), and lithiumbis(trifluoromethylsulfonyl)imid (Li(CF₃ SO₂)₂ N).

The above-mentioned salts show some serious drawbacks. Lithiumperchlorate has a tendency to explode in combination with some solventssuch as dioxolane. Compounds with fluorinated inorganic anions, such aslithium hexafluorophosphate, are of low thermal stability, and generateLewis acids by dissociation which are able to polymerized the solvent.Because of its arson content lithium hexafluoroarsenate isenvironmentally damaging, and generates carcinogenic by-products byreaction with lithium. Lithium bis(trifluoromethylsulfonyl)imide iscomparatively expensive.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an electrolyte for usein a galvanic cell which is non-explosive, stable, and ecologicallyharmless and a galvanic cell using the electrolyte.

This is achieved by an electrolyte comprising borates of the formulaABL₂, wherein A is lithium or a quarternary ammonium ion. The bidentatechelate ligand L is bonded to the central boron atom via two oxygenatom. In general chelate complexes of this type are of high thermalstability. There is little risk of an exothermic reaction with thesolvent or with lithium, because borates are no highly oxidizing agents.Therefore, the electrolytes in accordance with the invention arenon-explosive. Furthermore, the salts in accordance with the inventionare not able to form Lewis acids, which would polymerized the solvent.Therefore, the electrolytes are stable for a long time and can be usedover a wide temperature range. The salts in accordance with theinvention comprise no elements which are able to form toxic compoundsduring combustion. In general hydrolytic products of the salts areharmless and of low toxicity. Therefore, the electrolytes in accordancewith the invention are environmentally harmless.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 9 show cyclic voltammograms of the residual current of somesystems of the electrolyte of the disclosed invention:

FIGS. 10 to 12 show magnification of the anodic region for two systemsof the electrolyte of the disclosed invention; and

FIGS. 13 to 16 show galvanostatic cycling experiments of the electrolyteof the disclosed invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The conductivity of the electrolytes is increased by making electrolytesof the type according to the invention which contain a quarternaryammonium salt in addition to the lithium salt. For example,tetraalkylammonium salts with four alkyl groups (cf. example 2, and 3)or quarternary ammonium salts with spirane structure such asbispiperidenium salts can be used. Furthermore, the residues bounded tothe nitrogen atom may contain hereto atoms, as for example in the caseof the (2-ethoxyethyl) trimethylammonium ion. Furthermore, theconductivity of the electrolytes is increased by using mixtures of asolvent with high permittivity and a solvent with low viscosity. Data ofthis type of systems are shown in example 5.

The electrochemical properties of several salts in accordance with theinvention were investigated by the methods shown in a paper of Barthelet at. (J. Electrochem. Soc., Vol. 140, 6, (1993)). The examples 6, 7,9, 10, and 11 demonstrate electrolytes showing excellent cyclingbehaviour, if they comprise an organic carbonate as a solvent, forexample PC.

Two bidentate chelate ligands are linked to the central boron atom byoxygen atoms in the salts according to the invention. There is always anegative partial charge at the oxygen atoms, because oxygen is a highlyelectronegative element. It is advantageous, if the ligand comprises anaromatic system reducing the electron density respectivty the partialcharge, because in this way the negative charge is equal spread over theanion. The interaction between cation and anion is reduced both in solidand in solution. This results in a relative high conductivity andsolubility of the compounds in aprotic organic solvents.

The ligand should not be too large, because in this case anion mobilityis low resulting in a poor conductivity. Therefore, ligands areadvantageous, if they comprise an aromatic six-membered ring.

The delocalisation of the negative charge of the anion is increased, ifthe ligand comprises electronegative atoms which are able to reduce thecharge at the oxygen atoms by their inductive effect. In particular,this is achieved by using aromatic compounds with fluorine substituentsor heterocyclic aromatic compounds containing nitrogen atoms, forexample. The right column of table 1 shows the partial charge Q at theoxygen atoms of several said anions in units of the elementary charge e.Data were calculated by a semiempirical quantum-mechanical computation.Obviously, the partial charge at the oxygen atoms is significantlyreduced by the electronegative atoms fluorine and nitrogen.

The applicability of said electrolytes depends on conductivity, cyclingbehaviour stability against lithium, but also on compatibility with thecathode material. If highly oxidizing materials are used, it may beuseful to apply electrolytes containing aromatic anions with fluorinesubstituents or heterocyclic aromatic anions, because use of chelatecomplexes with electronegative atoms results in a decrease of the energyof the highest occupied molecular orbital (HOMO) and consequently in anincreased stability versus oxidation. Knowing the anodic potential limitof one electrolyte in accordance with the invention (cf. example 8), itis possible to estimate the anodic potential limit of similar structuredcompounds using a method of Yilmaz and Yurtsever (J. Electroanal. Chem.,Vol. 261, 105-112, (1989)). The HOMO energies of several said anions aregiven in Table 1.

Several salts in accordance with the invention are produced by simpleand inexpensive procedures, for example by crystallization from aqueoussolution (cf. examples 1 and 2) or by a precipitation reaction (cf.example 3). Furthermore, the chemicals used for the production oflithium bis[1,2-benzenediolato(2-)-O,O']-borate(1-), lithiumbis[salicylato(2-)]borate(1-), and tetraalkylammoniumbis[1,2-benzenediolato(2-)-O,O']borate(1-) are inexpensive, so thatproduction costs are low.

Formulas 1 to 7 show some structural formulas of compounds of the typeaccording to the invention.

Structural formula 1: ##STR1##

Empirical formula: LiB(C₆ H₄ O₂)₂

Systematic name: Lithium bis[1,2-benzenediolato(2-)-O,O']borate(1-)

Other names: Lithium bispyrocatecholatoborate, lithium biscatecholborate

Structural formula 2: ##STR2##

Empirical formula: LiB(C₇ H₄ O₃)₂

Systematic name: Lithium bis[salicylato(2-)]borate(1-)

Other name: Lithium bissalicylatoborate

Structural formula 3: ##STR3##

Empirical formula: LiB(C₆ H₃ FO₂)₂

Systematic name: Lithiumbis[3-fluoro-1,2-benzenediolato(2-)-O,O']borate(1-)

Structural formula 4: ##STR4##

Empirical formula: Li[C₆ H₂ F₂ O₂ ]

Systematic name: Lithiumbis[3,6-difluoro-1,2-benzenediolato(2-)-O,O']borate(1-)

Structural formula 5: ##STR5##

Empirical formula: LiB(C₆ O₂ F₄)₂

Systematic name: Lithiumbis[3,4,5,6-tetrafluoro-1,2-benzenediolato(2-)-O,O']borate(1-)

Structural formula 6: ##STR6##

Empirical formula: LiB(C₅ H₃ NO₂)₂

Systematic name: Lithium bis[2,3-pyridinediolato(2-)-O,O']borate(1-)

Structural formula 7: ##STR7##

Empirical formula: LiB(C₄ H₂ N₂ O₂)₂

Systematic name: Lithium bis[2,3-pyrazinediolato(2-)-O,O']borate(1-)

Another object of the invention is a galvanic cell comprising an anode,a cathode, and said electrolyte in accordance with the invention.Lithium, a lithium alloy or a Li⁺ intercalating carbon electrode can beused as anode (cf. examples 10 and 11). A Li⁺ intercalating electrodesuch as TiS₂, MoS₂ or vanadiumoxide can be used as cathode, where theequilibrium potential of the cathode should be lower than the anodicpotential limit of the electrolyte. Salts comprising aromatic anionswith fluorine substituents or heterocyclic aromatic anions should bepreferred to avoid anodic oxidation of the electrolyte, if highlyoxidizing cathode materials such as MnO₂, NiO₂ or CoO₂ are used. Theelectrolytes in accordance with the invention can be used both inprimary and secondary lithium cells.

                  TABLE 1                                                         ______________________________________                                        Ligand      Anion       E.sub.HOMO · eV.sup.-1                                                           Q · e.sup.-1                     ______________________________________                                        Catechol    B(C.sub.6 H.sub.4 O.sub.2).sub.2.sup.-                                                    -4,84       -0,244                                    3-Fluorocatechol                                                                          B(C.sub.6 H.sub.3 O.sub.2 F).sub.2.sup.-                                                  -5,16       -0,234                                    3,6-Difluorocatechol                                                                      B(C.sub.6 H.sub.2 O.sub.2 F.sub.2).sub.2 .sup.-                                           -5,50       -0,224                                    Tetrafluorocatechol                                                                       B(C.sub.6 O.sub.2 F.sub.4).sub.2.sup.-                                                    -6,03       -0,214                                    2,3-Dihydroxypyridine                                                                     B(C.sub.5 H.sub.3 NO.sub.2).sub.2.sup.-                                                   -5,20       -0,227                                    2,3-Dihydroxypyrazine                                                                     B(C.sub.4 H.sub.2 N.sub.2 O.sub.2).sub.2.sup.-                                            -5,65       -0,209                                    ______________________________________                                    

EXAMPLE 1

Synthesis of Lithium Bis[1,2-benzenediolato(2-)-O,O']borate(1-)

The salt is obtained by crystallization from concentrated aqueoussolution.

Equation of the Reaction:

    LiOH+B(OH).sub.3 +2 C.sub.6 H.sub.4 (OH).sub.2 →LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 +4H.sub.2 O

Procedure:

249.80 g (2.268 mole) catechol, 47.60 g (1.134 mole) LiOH.H₂ O, 70.13 g(1.1344 mole) boric acid, and 130 ml H₂ O are heated to 90°-95° Cels.under inert gas, where a homogeneous brown solution forms. The flask isallowed to cool down to room temperature in the course of about 5 hours,where the substance crystallizes in the shape of colourless squareplates. The flask is placed in a refrigerator for a night at +3° Cels.to increase the yield and then the mother lye is filtered off underinert gas applying the vacuum of a water jet pump. After drying for 39hours at room temperature in the vacuum of an oil pump colourlesscrystals polluted by some brown substance (from the mother lye) areobtained. The mass is 169.78 g. It is a hydrate of the compositionLiB(C₆ H₄ O₂)₂.2H₂ O (cf. analysis). The substance is dried at 100°-110°Cels., in the vacuum of an oil pump until weight constancy (39 h). Thecrystals decompose yielding a colourless powder.

Raw yield: 144,84 g, 0.6191 mole, 54.6%

Purification:

300 ml acetonitrile are added to 143.66 g of the crude product. Themixture is heated to mild boiling under inert gas, and acetonitrile isadded until a homogeneous solution forms. Colourless rectangular platesare obtained on cooling down overnight. The flask is placed in an icebath for 1 hour, and in an ice sodium chloride bath for 1 hour. Themother lye is removed by decanting under an inert gas flow.Recrystallization is performed three times. The mother lye is yellowishgreen on first recrystallization and colourless on thirdrecrystallization. Colourless needles of the composition LiB(C₆ H₄O₂)₂.2AN (cf. analysis) are obtained after drying at room temperaturefor 4 hours in the vacuum of an off pump. Subsequently, the substance isdecanted into a Sclenk flask, and dried in the vacuum of an oil pumpinside a glove box, where the temperature is raised to 150° Cels. bysmall steps. Drying is continued until no further loss in weight isfound, and no AN peak is detected by NMR spectroscopy at hightransmitter and receiver power. A colourless powder is obtained.

Yield: 65.46 g, 0.2798 mole, 76.9% per recrystallization

Analysis:

Gravimetry:

After decomposition of the hydrate there is a loss of mass of 24.80 g(1.3766 mole H₂ O), corresponding to 2.22 mole H₂ O per mole of thesalt. On final drying the weight of the AN solvate decreases from 88.58g to 65.46 g, corresponding to 23.12 g (0.5632 mole) AN, respectively2.01 mole AN per mole of the salt.

pH titration:

Formally, the bis[1,2-benzenediolato(2-)-O,O']borate(1-) anion iscomposed of two catechol units, and one B(OH)₄ ⁻ unit. The equivalencepoint was evaluated by titration with 0.5 n HCl at 50° Cels., and theboron content was calculated from it. Titration was carried out once forthe crude product and three times for the purified product. Thetheoretical boron content is 4.621%. A boron content was found of 4.611%(relative error 0.22%) for the crude product, and of 4.622% (relativeerror 0.02%) for the purified product.

NMR:

The NMR-spectrum of the purified product in DMSO-D₆ shows a singlet at6.5 ppm (TMS). This is a spectrum of the AA'BB' type, accidentally withequal chemical shifts of the protons as shown by comparativemeasurements using catechol dimethylether. In addition to the aromaticpeak the spectrum of pure catechol shows a peak at 8.7 ppm (TMS) whichis assigned to the OH groups. Neither the crude product nor the purifiedproduct shows this peak. In addition to the aromatic peak the crudeproduct dried at room temperature shows another peak at 3.7 ppm (TMS)which is assigned to the crystal water. A water content of 2.13 mole H₂O per mole of the salt is calculated for the hydrate from the integralcurve. Additional to the aromatic peak the AN solvate dried at roomtemperature shows a signal at 2.0 ppm (TMS) which is assigned to the CH₃group of acetonitrile. An AN content of 1.78 mole AN per mole of thesalt results from the integral curve.

Water analysis:

A sample of the purified product was dissolved in THF and a watercontent of 56 ppm in accordance with K. F. was determined by calculatingweight differences.

Thermal properties:

A sample of the substance is heated directly by the Bunsen time on asteel sheet. A greenish red coloured time as well as carbonizationappears. Catechol is easy to inflame by the Bunsen flame on a steelsheet, where it melts and burns up completely colouriug the flameyellow.

Solubility experiments in test glasses:

The substance is soluble in THF, 2-Me-THF, DME, PC, and of lowsolubility in dioxolane, diethylether, and toluene.

EXAMPLE 2

Synthesis of TetramethylammoniumBis[1,2-benzenediolato(2-)-O,O']borate(1-)

The salt is obtained by crystallization from aqueous solution.

Equation of the Reaction:

    N(CH.sub.3).sub.4 OH+B(OH).sub.3 +2 C.sub.6 H.sub.4 (OH).sub.2 →N(CH.sub.3).sub.4 B(C.sub.6 H.sub.4 O.sub.2).sub.2 +4 H.sub.2 O

Procedure:

86.98 g (0.7899 mole) catechol, 24.43 (0.39497 mole) boric acid, 360 ml10% aqueous solution of tetramethylammonium hydroxide, and 300 ml waterwere heated up to 80°-85° Cels. under inert gas, where a slightlyclouded yellowish brown solution forms. The hot solution is inoculatedand allowed to cool down to room temperature overnight, where colourlessquadrangular plates are obtained. Inoculation is necessary, because theproduct forms an oil otherwise. Inoculation crystals are obtained bytaking a sample of the solution, heating in a test glass with the helpof a heating fan, and cooling the solution to 0° Cels. in an ice bathwhile scratching with a glass rod.

The mother lye is decanted under an inert gas flow at room temperatureand the crude product is dried for 16 hours in the vacuum of an oilpump. Subsequently, drying is continued for 6.5 hours at 60°-70° Cels.in the vacuum of an oil pump. No change of the crystals is visible; theloss of weight is only 0.3%. The salt crystallizes free of crystalwater.

Raw yield: 67.39 g, 0.2238 mole, 56,6%

Purification:

64.94 g of the raw product and 100-150 ml acetone are heated to 50°-55°Cels. under inert gas resulting in a homogeneous brown solution which isallowed to cool down to room temperature, and inoculated. Colourlessrectangular flat needles are obtained. After standing overnight theflask is placed in an ice bath for 2 hours and in an ice sodium chloridebath for 45 minutes. The mother lye is decanted under an inert gas flow.Altogether, recrystallization is performed 5 times. After the fifthrecrystallization the mother lye is clear and colourless. The colourlesscrystals are dried in the vacuum of an off pump for four hours at roomtemperature, where they decompose yielding a colourless powder.Subsequently, the substance is dried for 3 hours at 50°-70° Cels. in thevacuum of an oil pump. No loss of weight is detected at this. Obviously,the crystals are an acetone solvate which already decomposes at roomtemperature in the vacuum of an oil pump.

Yield: 33.57 g, 0.1147 mole, 87.6% per recrystallization

Analysis:

NMR:

The NMR spectrum of the crude product in DMSO shows a peak at 3.0 ppm(TMS) which is assigned to the methyl groups, as well as a peak at 6.5ppm (TMS) which is assigned to the protons of thebis[1,2-benzenediolato(2-)-O,O']borate(1-) anion. Furthermore, somesmall peaks are found in the range of 6.5-6.9 ppm (TMS) which can beobviously attributed to impurities. H₂ O is not detected. The purifiedproduct shows only the two peaks at 3.0 and 6.5 ppm (TMS). The ratio ofintegrals agrees with the theoretical value within the accuracy inmeasurement

Solubility experiments in test glasses:

The substance is soluble in PC, dioxolane, and AN, scarcely soluble inDME, and THF, and of low solubility in toluene.

EXAMPLE 3

Synthesis of ButyltriethylammoniumBis[1,2-benzenediolato(2-)-O,O']borate(1-)

The salt is synthesized by precipitation of lithium chloride inacetonitrile.

Equation of the Reaction:

    N(C.sub.4 H.sub.9)(C.sub.2 H.sub.5).sub.3 Cl+LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 →N(C.sub.4 H.sub.9)(C.sub.2 H.sub.5).sub.3 B(C.sub.6 H.sub.4 O.sub.2).sub.2 +LiCl↓

Procedure:

5.49 g (23.47 mmole) lithium bis[1,2-benzenediolato(2-)O,O']borate(1-),4.53 g (23.38 mmole) butyltriethylammonium chloride and 30 mlacetonitrile are stirred in an Erlenmeyer flask at room temperatureinside a glove box. A colourless suspension of lithium chloride forms.Filtration yields a colourless precipitate of small crystals of lithiumchloride as well as a clear colourless filtrate. The flitrate isevaporated to dryness at room temperature in the vacuum of an oil pump,and the colourless solid substance is dried for 17 hours at roomtemperature.

Raw yield: 8.59 g, 22.29 mmole, 95.4%

Purification:

19 ml acetonitrile are added to 8.18 g of the substance under inert gasat room temperature. A clear colourless solution is obtained. Then 50 mldiethylether are added resulting in a slightly clouded solution.Filtration under inert gas results in a clear colourless solution aswell as in a small fraction of a colourless solid (probably LiCl).Storing for one night in a cold save (-25° Cels.) yields colourlessplates. The mother lye is decanted under an inert gas flow.Recrystallization is carried out twice. Then the substance is dried inthe vacuum of an oil pump at room temperature for two hours, and at50°-65° Cels. for 19 hours. The loss of mass is only 0.6%; no solvate isformed.

Yield: 6.92 g, 17.96 mmol, 92.0% per recrystallization

Analysis:

NMR:

As expected the NMR spectrum is a superposition of the spectra of purebutyltriethylammonium chloride, and of lithiumbis[1,2-benzenediolato(2-)-O,O']-borate(1-).

Melting point: 86°-87° Cels.

EXAMPLE 4

Synthesis of Lithium Bis[salicylato(2-)]borate(1-)

Equation of the Reaction:

    LiOH+B(OH).sub.3 +2 C.sub.7 H.sub.6 O.sub.3 →LiB(C.sub.7 H.sub.4 O.sub.3).sub.2 +4 H.sub.2 O

Procedure:

244.90 g (1.7731 mole) salicylic acid, 37.20 g (0.8860 mole) LiOH.H₂ O,54.82 g (0.8866 mole) boric acid, and 100 ml H₂ O are heated to mildboiling. A homogenous colourless solution forms. On cooling down to roomtemperature a voluminous colourless precipitate of small crystals isobtained which cannot be filtered. Crystals for inoculation are taken;heating to the boiling point is repeated; the flask is placed in a Dewarcontaining hot water, and inoculated. On cooling down a precipitate ofsmall crystals is obtained. After 2 days the flask is placed in arefrigerator for 7 hours at +10° Cels., and for 2 days at 0° Celsius.The flask is supplied with a connecting tube containing a glass filteras well as with a second flask, and the mother lye is filtered off at 0°Cels. applying the vacuum of a water jet pump. The substance is dried inthe vacuum of an oil pump for 7 hours at 0° Cels., end for approximately50 hours at room temperature. 197.76 g of product are obtained. Dryingfor 280 hours at 80°-180° Cels. reduces the weight to 184.76 g. Aftertaking a sample drying is continued for 93 hours at 170°-180° Cels.reducing the weight from 184.59 g to 183.15 g.

Raw yield: 183.15 g, 0.63194 mole, 71.2%

Purification:

179.58 g of the crude product and 850 ml acetonitrile are heated to mildboiling under inert gas. A slight cloudiness is obtained which does notvanish on raising the volume of the solution to 960 ml. The solution isfiltered under inert gas and cooled down to room temperature. A layer ofcolourless crystals forms at the wall of the flask. The flask is storedin a refrigerator for 3 hours at 0° Cels., and in a cold save for onenight at -25° Celsius. Then the mother lye is decanted under an inertgas flow. Altogether, recrystallization is performed three times. Onsecond recrystallization a cloudiness is visible again (filtration); onthird recrystallization the solution remains clear. Drying for 1.5 hoursat room temperature in the vacuum of an oil pump yields 151.84 g. Thendrying is continued for 171 hours until weight constancy in the vacuumof an oil pump, where the temperature is raised slowly from 80°-90°Cels. to 150°-155° Celsius. A colourless powder is obtained.

Yield: 118.11 g, 0.4073 mole, 86.4% per recrystallization

Analysis:

Gravimetry:

The mass decreases by 11.4 g (0.6350 mole H₂ O) on drying the crudeproduct at elevated temperature, corresponding to 1.00 mole H₂ O permole of the salt. The mass of the final product decreases by 33.73 g(0.8216 mole AN) on drying, corresponding to 2.02 mole AN per mole ofthe salt. The solvates are of the composition LiB(C₇ H₄ O₃)₂.H₂ O, andLiB(C₇ H₄ O₃).2AN.

pH-titration:

Formally the bis[salicylato(2-)]borate(1-) anion is composed of twosalicylic acid units and one B(OH)₄ ⁻ unit. Therefore, lithiumbis[salicylato(2-)]borate(1-) reacts acidic on hydrolysis in aqueoussolution The equivalence point was evaluated by titration at 70° Cels.using 0.1 n NaOH. Theoretically, the expected quantity of lye is 1 moleNaOH per mole of the salt. Titration was carried out once for the crudeproduct and three times for the purified product. A consumption of lyewas found of 0.996 mole NaOH per mole of the salt for the crude product,and of 0.9992 mole NaOH per mole of the salt for the purified product.

NMR:

In DMSO-D6 the crude product dried at room temperature shows a multipletwith its centre at 7.3 ppm (TMS) which is assigned to the aromaticresidue, as well as a singlet at 3.5 ppm which is referred to water. Awater content of 1.01 mole H₂ O per mole of the salt results from theintegral curve. The crude product dried at elevated temperature stillshows a small water peak which is not to evaluate quantitatively,because commercially available DMSO as well as TMS always show a smallcontent of residual water as proved by comparative measurements usingthe pure solvent. The product recrystallized from AN and dried at roomtemperature shows a singlet at 2.0 ppm (TMS) in addition to the aromaticmultiplet which is assigned to AN bonded in the solvate. An AN contentof 1.70 mole per mole of the salt results from the integral curve. Thevalue which is somewhat to low probably refers to loss by evaporation onfilling the tube inside a glove box. The purified product dried atelevated temperature only shows a multiplet at 7.3 ppm (TMS). At a hightransmitter and receiver power a small peak is perceptible at 2.0 ppmwhich is close to the detection limit of the method and cannot beevaluated quantitatively. A comparative spectrum of salicylic acid wasrecorded showing a broad peak at 12.4 ppm (TMS) which is assigned to theprotons of the COOH and OH groups. This peak is not detectable in theNMR spectrum of lithium bis[salicylato(2-)]borate(1-).

Water Analysis:

A sample of the substance was dissolved in PC and the water content inaccordance with K. F. was determined by calculating weight differences.A H₂ O content was found of 2200 ppm for the crude product, and of 64ppm for the purified product.

Thermal properties:

A sample of the dried product is heated directly by the Bunsen flame ona stainless steel sheet. The substance burns colouring the flame greenand red; carbonization occurs.

Solubility experiments in test glasses:

The substance is soluble in THF, 2-Me-THF, DME, PC, dioxolane, andacetone, and of low solubility in diethylether. However, colourlesscrystals (solvate) form in THF and 2-Me-THF on standing for a long time(some hours up to some days). The substance tends to form supersaturatedsolutions.

EXAMPLE 5

Conductivity

The conductivity of some electrolytes was measured using a smallmeasuring cell and a commercially available bridge. The temperature wasmeasured with a mercury thermometer. The specific conductivities, asshown in table 2, are measured at arbitrary concentrations. This are notχ_(max) -values. The values are not very precise; they are meant toserve for a rough estimation of attainable conductivity.

                  TABLE 2                                                         ______________________________________                                        System                 υ °C..sup.-1                                                            κ mS.sup.-1 cm                          ______________________________________                                        1.231 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in PC                                                     27       0,6                                       1.184 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in THF                                                    29       1,4                                       0.899 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in DME                                                    26       1,7                                       0.595 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in 2-Methyl-THF                                           29       0,4                                       1.11  molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in a mixture                                              25       3,0                                       45.7  weight % PC und 54.3 weight % DME                                       0.519 molal N(CH.sub.3).sub.4 B(C.sub.6 H.sub.4 O.sub.2).sub.2                                           17d      2,8                                       0.293 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in PC                          0.561 molal N(CH.sub.3).sub.4 B(C.sub.6 H.sub.4 O.sub.2).sub.2                                           18.5     5,1                                       0.271 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in a mixture of                60.4  weight % THF and 39.6 weight % PC                                       0.400 molal N(CH.sub.3).sub.4 B(C.sub.6 H.sub.4 O.sub.2).sub.2                                           23d      5,5                                       0.555 molal LiB(C.sub.6 H.sub.4 O.sub.2).sub.2 in a mixture of                52.8  weight % PC and 47.2 weight % DME                                       0.713 molal LiB(C.sub.7 H.sub.4 O.sub.3).sub.2 in DME                                                    27       0,8                                       0.775 molal LiB(C.sub.7 H.sub.4 O.sub.2).sub.2 in PC                                                     29       0,4                                       0.840 molal LiB(C.sub.7 H.sub.4 O.sub.2).sub.2 in a mixture                                              26       1,7                                       59.8  weight % PC and 40.2 weight % DME                                       ______________________________________                                    

EXAMPLE 6

CV of the Residual Current

In each case 5 cyclic voltammograms were recorded in series using astainless steel electrode with an area of 0.503 cm². The potential wasswept from the open-circuit potential to 0 mV vs. Li/Li⁺, and back tothe open-circuit potential again at a potential sweep rate of 20 mV/s.Lithium bis[1,2-benzenediolato(2-)-O,O']borate(1-) was investigated inorganic solvents, and in mixtures of organic solvents as well as withtetraalkylammonium bis[1,2-benzenediolato(2-)-O,O']borates(1-) as anadditive. The cyclic voltammograms of the residual current of somesystems are shown in FIG. 1 to FIG. 4. The curves are scarcelystructured, and don't show any reduction peak which contrasts markedlywith the residual current. No reduction of the tetramethylammonium ionsis visible (cf. FIG. 1, and 2).

EXAMPLE 7

CV for Lithium Deposition

In each case 5 cyclic voltammograms were recorded in series using astainless steel electrode with an area of 0.503 cm². The experimentswere carried out without exchanging the working electrode immediatelyafter having finished the cyclic voltammograms of residual current. Thepotential was swept from the open-circuit potential to -500 mV vs.Li/Li⁺, then to + 3000 mV vs. Li/Li⁺, and finally back to theopen-circuit potential at a potential sweep rate of 20 mV/s. Theefficiency was evaluated by numerical integration in the I-t graph. FIG.5 to 9 show the cyclic voltammograms of some systems. The meanefficiency of 5 cycles is shown on the right side of the CVs.

Systems containing PC as a solvent or mixtures of PC and ethers showrelatively high efficiencies, as can be seen in the shown cyclicvoltammograms, whereas systems containing pure DME, 2-Me-THF, and THF(not shown here) as a solvent yield only low efficiencies.Tetramethylammonium ions scarcely influence the cycling behaviour.

EXAMPLE 8

Anodic Potential Limit

In each case 5 cyclic voltammograms were recorded in series to evaluatethe anodic potential limit using a stainless steel electrode with anarea of 0.503 cm². The potential was swept at a sweep rate of 20 mV/sfrom the open circuit potential to 4000 mV vs. Li/Li⁺, then to 0 mV vs.Li/Li⁺, and finally back to the open circuit potential. A typical CV isshown in FIG. 10. FIGS. 11, and 12 show a magnification of the anodicregion for two systems. The anodic potential limit of about 3.6 V vs.Li/Li⁺ depends scarcely on the composition of the solution.

EXAMPLE 9

Cycling on Stainless Steel

Galvanostatic cycling experiments were carried out at a charge densityof 100 mC/cm², and at a current density of 1 mA/cm² to evaluate cyclingefficiency using a stainless steel electrode with an area of 0.503 cm².Some examples are shown in FIGS. 13-15. In solutions containing PC theefficiency is relatively high at the former cycles, comes up to amaximum, and decreases finally to small values. Lithium deposits in theshape of grey needles which do not dissolve on exposure to theelectrolyte for some days. Hence the loss of efficiency is referred to aloss of contact of lithium. Cycling efficiency is low in solutionscontaining solely DME, THF, or 2-Methyl-THF as a solvent. No significantinfluence of tetraalkylammonium salts on cycling efficiency is observed.No visible change of the investigated solutions (formation of gasbubbles or colouration) was observed.

EXAMPLE 10

Cycling on Carbon

A cycling experiment was carried out at a charge density of 100 mC/cm²,and at a current density of 1 mA/cm² using a glassy carbon electrodewith an area of 0.0707 cm² (3 mm diameter). The potential limit wasfixed to 1500 mV vs. Li/Li⁺. A solution was arbitrarily chosen as anelectrolyte which contained 0.561 mol/kg tetrmethylammoniumbis[1,2-benzenediolato(2-)-O,O']borate(1-), and 0.271 mol/kg lithiumbis[1,2-benzenediolato(2-)-O,O']borate(1-) in a mixture containing 39.6weight % PC, and 60.4 weight % THF. Altogether, 1100 cycles were runwith stops of 5 minutes after cycle 100, 145 minutes after cycle 300, 3minutes after cycle 400, and 94 minutes after cycle 700. The cyclingefficiency increases continually at this. FIG. 16 shows the former 100cycles, and FIG. 17 the final 400 cycles. The curves show the expectedappearance. The efficiency is low in the former cycles, because the meanpart of charge is consumed by irreversible processes (e.g. formation ofa covering layer). The efficiency increases to high values at increasingnumber of cycles. No change of the electrolyte (formation of gas bubblesor coloration) was observed.

EXAMPLE 11

Cycling on Lithium

A half cell was used to evaluate cycling efficiency. It consists of astainless steel working electrode covered with a 0.1 mm thick layer oflithium which was pressed onto the surface, a lithium foil as a counterelectrode, a separator and a cell-container made of glass. The distancebetween the electrodes was about 3 mm, the area of the electrode 1 cm²,and the volume of the electrolyte 3 or 4 cm³. The separator (Celgard2400) was made of polypropylene. The solution contained 0.400 mol/kgtetramethylammonium bis[1,2-benzenediolato(2-)-O,O']borate(1-) and 0.555mol/kg lithium bis[1,2-benzenediolato(2-)-O,O']borate(1-) in a mixtureof 52.8 weight % PC and 47.2 weight % DME. Cycling was carried out at acurrent density of 0.5 mA/cm² on charging, and at a current density of1.0 mA/cm² on discharging, as well as at a DOD of 22.6%. The pressed-oncharge was 101.62 C. Grey dendrites formed on the counter electrodeduring first cycle. No formation of gas bubbles, and no colouration ofthe electrolyte was observed. At the end of cycling the space betweenthe electrodes was filled with a flurry of lithium. 4.90 cycles wereachieved. Various modifications in the structure, function, steps andfeatures of the disclosed invention may be made by one skilled in theart without departing from the scope and extent of the claims.

I claim:
 1. An electrolyte for use in a galvanic cell comprising atleast one compound represented by the formula ABL₂, wherein A is lithiumor a quarternary ammonium ion, B is boron, and L is a bidentate ligand,which is linked to the central boron atom by two oxygen atoms, with theproviso that the electrolyte comprises at least one lithium salt.
 2. Theelectrolyte in accordance with claim 1 comprising at least one compoundrepresented by the formula ABL₂, which is dissolved in an organicsolvent, a mixture of organic solvents, one or more polymers, or amixture of one or more polymers and at least one organic solvent.
 3. Theelectrolyte in accordance with claim 1, wherein the quarternary ammoniumion is represented by the formula NR₄, NR₃ ^(A) R^(B), NR₂ ^(C) orNR^(C) R^(D), where R, R^(A), R^(B), R^(C) and R^(D) represent organicresidues.
 4. The electrolyte in accordance with claim 1, comprising atleast one compound represented by the formula ABL₂, which is dissolvedin propylene carbonate or butylene carbonate, or in a mixture ofsolvents comprising at least propylene carbonate, butylene carbonate orethylene carbonate.
 5. The electrolyte in accordance with claim 1,wherein the ligand L comprises at least one aromatic group.
 6. Theelectrolyte in accordance with claim 5, wherein the ligand L comprisesat least one aromatic six-membered ring.
 7. The electrolyte inaccordance with claim 6, wherein the aromatic six-membered ringcomprises at least one fluorine atom linked to the ring, and at leastone nitrogen atom within the ring.
 8. The electrolyte in accordance withclaim 7, wherein the ligand L is represented by the formula C₆ H₃ FO₂,C₆ H₂ F₂ O₂, C₆ HF₃ O₂ or C₆ F₄ O₂.
 9. The electrolyte in accordancewith claim 7, wherein the ligand L is represented by the formula C₅ H₃NO₂ or C₄ H₂ N₂ O₂.
 10. The electrolyte in accordance with claim 6,wherein the ligand L is represented by the formula C₆ H.sub.(4-x) R_(x)O₂, where x is an integer from 0 up to 4, and R is an alkyl residue. 11.The electrolyte in accordance with claim 1, wherein the compound ABL₂ islithium bis[1,2-benzenediolato(2-)-O,O']borate(1-) represented by theformula LiB(C₆ H₄ O₂)₂ or a tetraalkylammoniumbis[1,2-benzenediolato(2-)-O,O']borate(1-) represented by the formulaNR₄ B(C₆ H₄ O₂)₂ or NR₃ ^(A) R^(B) B(C₆ H₄ O₂)₂, where R, R^(A), andR^(B) are alkyl residues.
 12. The electrolyte in accordance with claim1, wherein the compound ABL₂ is lithiumbis[1,2-benzenediolato(2-)]borate(1-) represented by the formula LiB(C₆H₄ O₂)₂.
 13. The electrolyte in accordance with claim 1, wherein thecompound ABL₂ is lithium bis[salicylato(2-)]borate(1-) represented bythe formula LiB(C₇ H₄ O₃)₂.
 14. The electrolyte in accordance with claim1 wherein the at least one compound represented by the formula ABL₂ isdissolved in an organic solvent, a mixture of organic solvents, one ormore polymers, or a mixture of one or more polymers and at least onorganic solvent, and wherein the ligand L comprises at least onearomatic group.
 15. A galvanic cell comprising a cathode, an anode andan electrolyte which comprises at least one compound represented by theformula ABL₂, wherein A is lithium or a quaternary ammonium ion, B isboron, and L is a bidentate ligand, which is linked to the central boronatom by two oxygen atoms, with the proviso that the electrolytecomprises at least one lithium salt.
 16. The galvanic cell according toclaim 15 wherein the at least one compound represented by the formulaABL₂ is dissolved in propylene carbonate or butylene carbonate, or in amixture of solvents comprising at least propylene carbonate, butylenecarbonate, or ethylene carbonate.
 17. The galvanic cell according toclaim 15 wherein the ligand L comprises at least one aromatic group. 18.The galvanic cell according to claim 15 wherein the ligand L isrepresented by the formula C₆ H₃ FO₂, C₆ H₂ F₂ O₂, C₆ HF₃ O₂, or C₆ F₄O₂.
 19. The galvanic cell according to claim 16 wherein the ligand L isrepresented by the formula C₅ H₃ NO₂ or C₄ H₂ N₂ O₂.
 20. The galvaniccell according to claim 15 wherein the compound ABL₂ is lithiumbis[1,2-benzenediolato(2-)]borate (1-) represented by the formula LiB(C₆H₄ O₂)₂.